What was found
AT FL 390, NR 2 ENGINE QUIT RESPONDING TO THROTTLE MOVEMENT. PARAMETERS WERE NORMAL BUT CLOSE TO IDLE POWER. DESCENDED TO FL 290, NORMAL OPERATION RETURNED PASSING FL 320. FLEW REST OF FLIGHT AT FL 300. TEMP AT FL 390 WAS -55C. SUMPED ALL FUEL TANKS, NO EXCESSIVE WATER CONTAMINATION FOUND, CHECKED FUEL HEATER SYS FOR PROPER OPERATION, IAW AMM 73-15-00-05, OPS NORMAL. CHECKED PS4 LINE AND FOUND IT LOOSE AT THE HMU, PURGED INSTALLED AND TIGHTENED PS4 LINE. RAN ENGINE IAW JRH AT VARIOUS POWER SETTING, OPS CHECKED NORMAL.